A conventional CD-1 mouse colony, which was suspected to harbor low incidence levels of MHV and mites, was used to compare two different exposure routes for sentinel animals. For the last eight months sentinel mice, not exposed to dirty bedding, were negative for mites and MHV. The objectives of this study were to compare two different sentinel animal exposure routes and to determine whether these agents were present in this colony. Eight cages of experimental sentinel mice, containing 12 mice/cage, housed without filter bonnets, were placed in the room on four different racks. Twice each week, four cages received a composite sample of bedding used previously by mice in the room. The remaining four cages received fresh unused bedding. Sentinel mice were monitored at monthly intervals. After four months, mice from two cages which received used bedding seroconverted to MHV and ore cage was positive for mites. Three weeks later, all four cages of mice which received used bedding were positive for MHV and three were positive for mites. In contrast, only two of four cages of mice which received fresh bedding were positive for MHV and all were negative for mites. These findings indicate the importance of exposing sentinel mice to used bedding and that MHV and mites may go undetected for several months in a mouse colony when the incidence levels are less than 10% and where strict sanitation practices are used. It was concluded that MHV and mites are more easily transmitted to sentinel mice by exposing sentinel mice to dirty bedding. As a result of these findings, all sentinel animals are exposed to dirty bedding and we have improved our sentinel animal program.